There are some known processes for individualizing or dicing a semiconductor substrate into a plurality of element chips, including for example, a blade-dicing process using a mechanical cutting technique with a blade cutter, a laser-dicing process using a laser processing technique, and a plasma-dicing process using a plasma processing technique. The dicing processes are performed on the semiconductor substrate while being adhered on a holding sheet held on a frame, in order to facilitate handling of a thin substrate to be processed and/or a plurality of the elemental chips after being diced.
Unlike the blade-dicing process and the laser-dicing process, the plasma-dicing process has an advantage achieving a high processing efficiency even if the number of the element chips is enormous since it collectively processes the whole substrate surface. However, since the plasma-dicing process is performed to dice the substrate through a plasma exposure across the whole substrate surface, it requires a coating on element regions of the substrate and a mask having a pattern uncovering dicing regions of the substrate. The mask may be formed by a photolithography technique or a laser-grooving technique.
The mask formation by the photolithography technique is achieved by heating a resist solution applied on the substrate surface over 90 degrees C. or greater for drying to form the resist layer, exposing and developing the resist layer for patterning thereon.
The mask formation by the laser-grooving is achieved by applying a resin material solution on the substrate surface to form the resin layer, irradiating a laser beam onto a portion of the resin layer, and removing the irradiated portion of the resin layer for patterning thereon. The masking material used for the laser-grooving may include polyimide and polyvinyl alcohol, for example.
For example, Patent Document 1 (JP 2014-513868 A or WO 2012/125560 A2) discloses the plasma-dicing process using the resist mask. Also, Patent Document 2 (JP 2014-523112 A or WO 2012/173768 A2) discloses the plasma-dicing process using the resist layer formed by the laser-grooving step.
In the plasma-dicing process, the mask is formed before or after the substrate is adhered on the holding sheet. If the substrate is relatively thick causing less warpage or crack and easy to be handled by itself, then the mask may be formed on the substrate as a typical wafer process before the substrate with the mask thereon is adhered onto the holding sheet. However, the thicker substrate requires a longer time of the plasma process for individualization (dicing), and also requires the thicker mask for withstanding such a longer time of the plasma process, so that the production yield is likely reduced. For this reason, it is preferable to make the substrate thinner by grinding it and to adhere the thinner substrate onto the holding sheet before the plasma-dicing process, which shortens the plasma processing time and also thins the mask. On the other hand, when the mask is formed on the thinner substrate and then the substrate is adhered on the holding sheet, the thinner substrate is more susceptive to warpage and crack and more difficult to be handled, therefore the production yield is likely reduced. Thus, if the substrate may be adhered on the holding sheet before the mask is formed, it would be expected to produce the substrate which is less susceptive to be damaged, thereby to improve the productivity and the production yield. But this causes drawbacks as follows.
The holding sheet typically used in the dicing process is made of thermoplastic resin which has less heat tolerance. Therefore, if the substrate is processed at a high temperature which may soften the holding sheet when forming the mask of the resist or resin, the holding sheet undergoes deformation (expansion/shrinkage and curvature) which prevents the wafer from being maintained in a flat configuration. The deformed wafer deteriorates the patterning accuracy in the mask formation step, and also causes the holding sheet and the substrate to be insufficiently cooled in the plasma processing step, which eventually reduces the production yield of the element chips as the finished products.
In the meanwhile, when the mask is formed by the photolithography technique, if the liquid resist is not dried enough due to shortage of the heating step in the mask formation, the patterning accuracy of the resist mask in an exposing step is deteriorated. Also, so-called a mask burning or a resist burning is caused to damage the resist mask with deformation by heat in the plasma processing step. Similarly, when the mask is formed by the laser grooving step, the mask burning is caused in the plasma processing step due to the insufficient heating in the mask formation.